I've worked on over 20 software contracts and never once did the contract contain a clause that forced me to assign copyright agreement to the person paying for the software. Note that this only applies to contractors, not exempt employees.

http://www.developerdotstar.com/mag/articles/daniels_softwar...

This is the other extreme - which I didnt mention in the op.

I agree that copying code is wrong - but what about a good idea that you come up with, or a neat design or technique in code?

It seems unfair given that doctors, scientists, marketing and many other professionals can keep a record of their work. Maybe not reams of data, but something. (I am not 100% sure of the extent they are allowed to). All art professionals have portfolios.

Is your brain just a vessel to be washed clean before it is rented out again?

Again, I am not talking of secret algorithms that would break a company, not someone else's work.

I can never really agree to such a clause, though. In the most extreme case it would mean I could never take on another contract, because I already used up all my programming tricks in the first one. For example I might have employed the MVC pattern, will I be able to ever use it again?